1. Field
This patent specification describes a switching regulator, and more particularly, a switching regulator capable of efficient control at control mode change.
2. Background Art
Recently, energy efficiency has been actively promoted as an environmental protection measure. For portable devices using batteries, such as mobile phones, digital cameras, and so on, energy efficiency is especially important in order to prolong battery life. Generally, to achieve energy efficiency, power consumption of the electronic device is lowered and unnecessary power consumption is reduced by improving the efficiency of a power circuit of the electronic device.
As the power circuit, used in a compact electronic device, a non-insulated type switching regulator employing an inductor is widely used. There are two main control modes to control the switching regulator. One is PWM (pulse width modulation) control mode and the other is VFM (variable frequency modulation) control mode.
In the PWM control mode, a duty cycle of a clock pulse having a constant frequency is changed to maintain an output voltage with a constant voltage value. By contrast, in the VFM control mode, a frequency of a clock pulse having a constant pulse width is changed in accordance with a deviation of the output voltage from a predetermined voltage.
In the PWM control mode, the switching transistor is switched on/off at a constant frequency even at a relatively light load, in which a small current flows into a load. In this light load condition, the performance efficiency of the switching regulator decreases. By contrast, in the VFM control mode, a frequency of a switching signal to switch the switching transistor is changed in accordance with the load condition. As a result, large noise and ripple for the device may be generated in comparison with the switching transistor using PWM control mode. However, the switching regulator using the VFM control mode can achieve higher performance efficiency than the performance efficiency using the PWM control mode under the relatively light load condition.
For the reason described above, both PWM and VFM control modes may be used by switching the two control modes in accordance with the load condition to obtain high performance efficiency in the power system in a wide range of load conditions, from light load to heavy load.
Generally, to detect the load condition, a detection resistor to detect an output current is connected between a power terminal that receives an input voltage and an output terminal. Then, an output current from the output terminal to the load is detected. However, in this method, as the output current increases, power consumption at the detection resistor increases. This power consumption is a large penalty for a compact electronic device using a battery.
Another device may use an indirect method that employs an error amplifier to detect an output voltage level instead of using the detection resistor to detect the output current. The error amplifier includes an integrating circuit to remove an effect of a ripple component superimposed on the output voltage. The integrating circuit is usually included as a phase compensation circuit of the error amplifier. Further, the integrating circuit is optimized to an operating frequency of the normal PWM control mode. Therefore, when the operating frequency becomes low or some pulses are removed from the pulse signal of the PWM control mode, the following problem occurs similar to the VFM control mode.
Specifically, immediately after start of a switching operation, the integrating circuit functions efficiently. However, when switching stops due to, for example, the removal of some pulses from the pulse signal of the PWM control mode, the output voltage of the error amplifier becomes 0 v or power supply voltage. As a result, the output signal of the integrating circuit may not be an efficient signal to detect the load current. For this reason, in the VFM control mode, the output voltage of the error amplifier cannot follow the load current and cannot be kept at a constant voltage value. Consequently, it is more difficult to set the load current accurately when the control mode changes in comparison with the method that uses the detection resistor to detect the output current.